Representatives of a device having a capacitive impedance include an image pickup device and a charge transfer device used as a delay device in a signal processing circuit.
The charge transfer device includes a group of devices laid on a semiconductor substrate, being separated away from the substrate by an insulation layer. The impedance of the charge transfer device can be approximated by an electrostatic capacitance. An operation to drive the charge transfer device raises a problem that a large current is consumed in a process to charge electric charge to the device and a process to discharge electric charge from the device. An operation to drive the charge transfer device at a higher speed consumes an even larger current in a process to charge electric charge to the device and a process to discharge electric charge from the device.
A variety of techniques have each been conceived as a solution to the problem. (For more information on the techniques, the reader is suggested to refer to documents such as Japanese Patent Laid-open No. Hei 1-303756 used as document 1, Japanese Patent Laid-open No. Hei 1-303757 used as document 2, Japanese Patent Laid-open No. Hei 1-303758 used as document 3 and Japanese Patent Laid-open No. Hei 11-98416 used as document 4.)
For example, documents 1 to 3 propose a configuration for reducing the power consumption of a charge coupled device by making use of an LC resonance circuit. For example, a configuration shown in FIG. 23 includes a variable-capacitance device C4 denoted by reference numeral 913 in addition to a resonance circuit employing an inductor L3 denoted by reference numeral L903 and a capacitor C3 denoted by reference numeral C903. The variable C4 is an adjustment capacitor for correcting capacitance variations of a charge coupled device 901. On the other hand, a configuration shown in FIG. 24A includes switches SW914, SW915 and SW916 as well as a parallel resonance circuit, which employs a charge transfer device 901 and an inductor L2 denoted by reference numeral L902. The switch SW916 is provided in the parallel resonance circuit whereas the switches SW914 and SW915 connect the parallel resonance circuit to power supplies.
The three switches SW914, SW915 and SW916 employed in the configuration shown in FIG. 24A are controlled to alternately switch a resonance circuit and a charging period so that currents flowing from the power supplies during the resonance period are reduced. In this way, reduction of the power consumption can be implemented. The resonance period is a portion of a period denoted by reference numeral 917 in FIG. 24B. In the resonance period, a signal φ15 is on. The charging period is also a portion included in the period 917 shown in FIG. 24B as a portion in which the signal φ15 is off. During the charging period, electric charge is charged from the power supplies to the charge transfer device.
A configuration according to the technique disclosed in document 4 employs a switch circuit including switches SW0, SW1 and SW2 denoted by reference numerals 909, 910 and 911 respectively as shown in FIG. 25A. The switch circuit is used or connecting a charge transfer gate electrode provided at the front stage of a charge coupled device to a charge transfer gate electrode provided at the rear stage of the charge coupled device. In accordance with the technique, paths of clock signals for driving the charge transfer gate electrodes are cut off after the charge transfer gate electrode provided at the front stage is driven but before the charge transfer gate electrode provided at the rear stage is driven.
In this case, a period for which a clock signal φ3 is set at 1 as shown in FIG. 25B is a period right after the charge transfer gate electrode provided at the front stage of the charge coupled device 908 is driven but before the charge transfer gate electrode provided at the rear stage is driven. During this period, a clock signal φ1 is set at 0 and a clock signal φ2 is also set at 0 for respectively cutting off the switches SW1 and SW2 for driving the charge transfer gate electrode provided at the front stage and the charge transfer gate electrode provided at the rear stage. With φ3=1, however, the switch SW2 is turned on. That is to say, by executing control to turn on the switches SW1 and SW2 but conversely turn off the switch SW3 or vice versa, the charge transfer gate electrode provided at the front stage and the charge transfer gate electrode provided at the rear stage are connected to each other with a timing determined in advance during a transfer of electric charge.
By executing such control, some of electric charge accumulated in the charge transfer gate electrode provided at the front stage is transferred to the charge transfer gate electrode provided at the rear stage allowing the transferred charge to be recycled. It is thus possible to reduce the amount of reactive energy discharged from and charged to the capacitors of the charge transfer gate electrode provided at the front stage of the charge coupled device and the charge transfer gate electrode provided at the rear stage of the charge coupled device. As a result, the power consumption can be decreased.
In addition, Japanese Patent Laid-open No. Hei 5-122625 used as document 5 proposes a 2-phase resonance drive circuit shown in FIG. 26A. As shown in the figure, a capacitive load 920 is equivalent to a capacitive load 921 shown in FIG. 26B. Thus, the circuit shown in FIG. 26A is equivalent to a circuit shown in FIG. 26B. The 2-phase resonance drive circuit shown in FIG. 26B includes a resonance loop having a configuration connected to the ground (GND). By setting a resonance state of a resonance circuit employing an electrode capacitor, a current is held in the resonance loop. Thus, the power consumption of the entire circuit can be reduced. The power consumed by the electrode capacitor of the conventional resonance circuit is a problem. In the case of the 2-phase resonance drive circuit, however, a current is held in the resonance loop so that the power consumption can be reduced substantially.
Nevertheless, the 2-phase resonance drive circuit is controlled by a signal including higher harmonic components of odd orders as is the case with a signal having a rectangular waveform. Thus, the 2-phase resonance drive circuit still has a problem that the power consumption increases due to the higher harmonic components.
In addition, in accordance with a technique disclosed in Japanese Patent Laid-open No. Hei 5-122619 used as document 6, an operation to supply one of 2-phase clock signals is delayed from an operation to supply the other 2-phase clock signal by at least one period of the clock signal as shown in FIG. 26C in order to avoid a phenomenon in which a transfer of electric charge fails due to the fact that the first generated driving power of the driving waveform is insufficient. The insufficient driving power is a problem peculiar to the CCD driving. By delaying the operation to supply one of 2-phase clock signals from the operation to supply the other 2-phase clock signal as described above, at least a pulse of one of the clock signals is excessively supplied to assure a driving level contributing to the CCD transfer.